A. Technical Field
The present invention relates to a process for production of hydrogel particles and a process for cutting of a high-concentration hydrogel sheet. Specifically, the present invention relates to: a process for production of a particulate hydrogel comprising the steps of cutting and disintegrating a hydrogel sheet of a water-absorbent resin (having beforehand been produced in the shape of a sheet) in a process for production of water-absorbent resin particles; and a process for cutting of a high-concentration hydrogel sheet which process is applicable to the above process for production of a particulate hydrogel.
B. Background Art
An art for production of hydrogel particles, including the steps of producing a sheet of a hydrogel of a water-absorbent resin and then disintegrating this hydrogel sheet, is known as an art for production of water-absorbent resin particles which are widely utilized for such as absorbing materials for absorbent products such as disposable diapers.
In a patent document 1 below, there is proposed a process for continuous production of a high-concentration hydrogel sheet including the steps of: supplying an aqueous solution, containing a monomer component such as acrylic acid, onto an endless belt; and then polymerizing the monomer component in a state left static. There is also disclosed an art for obtaining hydrogel particles by disintegrating the high-concentration hydrogel sheet with a shearing type primary crusher or cutting and shearing mill. The above patent document states that a screen type disintegrator, which has a screen and makes the disintegration by shearing between a fixed blade and a rotary blade, is favorable as the disintegrator. The resultant hydrogel particles are dried or pulverized, whereby water-absorbent resin particles are obtained. The high-concentration hydrogel sheet does not form into a flat and smooth sheet, but is in a form such that dispersion in thickness with places or thicknesswise undulating or waving is caused by foaming-expansion due to boiling during the polymerization and by subsequent shrinkage. The above patent document states that the high-concentration hydrogel sheet of such a form has an advantage such that the disintegration is so easy to carry out that the water-absorbent resin particles are easy to produce.
In a patent document 2 below, there is disclosed an art for production of a small-pieces-shaped water-soluble polymer gel including the steps of: cutting a sheet of a water-soluble polymer gel into the shape of rectangles with a pair of roller type cutters; and then cutting them with a rotary blade and a fixed blade also in directions crossing at right angles to the roller type cutters. The fixed blade is disposed just under the rectangular sheets being discharged from the pair of roller type cutters. The rotary blade is disposed on the outer periphery of a columnar. revolution body along its axial directions and interposes the rectangular sheets between the rotary blade and the fixed blade with the revolution of the revolution body to thereby cut the rectangular sheets. The above patent document states that even a hard polymer gel can easily be formed into small pieces and that the cohesion between these small pieces also occurs little.
In a patent document 3 below, there is disclosed an art for obtaining rectangular gel pieces including the steps of: running a continuous sheet of a hydrous polymer gel; and, in a nip clearance between a cutting roll and an opposite roll (which are disposed on the upside and downside of the continuous sheet), slitting the continuous sheet lengthwise by a disk-shaped cutter (provided to the cutting roll) to divide the continuous sheet widthwise, and cutting the continuous sheet off lengthwise every definite interval in such a manner that it is scraped off by a rectangular-plate-shaped rotary blade which is provided to the same cutting roll and extends in its axial directions. It is also disclosed that the resultant gel pieces are, for example, finely pulverized, whereby water-absorbent resin particles are obtained. It is stated that the gel pieces are successfully obtained even from a soft polymer gel sheet without its adhesion to the cutting blades.                [Patent Document 1] JP-A-212204/2002 (Kokai)        [Patent Document 2] JP-A-110510/1986 (Kokai)        [Patent Document 3] European Patent Specification No. 0827443B1        
As to the prior arts for cutting of the hydrogel sheet, the adhesion of the hydrogel to such as the cutting blades cannot sufficiently be prevented. Because of the adhering and clogging phenomena of the hydrogel, the operation has been stopped, or it has been necessary to often carry out such as operation of removing the adhered materials.
Particularly, in the case of such a high-concentration hydrogel sheet as disclosed in the patent document 1 above, because, as aforementioned, this sheet has the dispersion in thickness or the thicknesswise undulating or waving and further because this sheet has very strong adhesion or tackiness, there is a problem that this sheet tends to adhere to the cutter or that the cut pieces tend to cohere again.
In the art of the patent document 1 above, the high-concentration hydrogel sheet is disintegrated with the shearing type primary crusher or cutting and shearing mill. However, if an attempt is made to disintegrate the high-concentration hydrogel sheet while continuously supplying this sheet (being continuously produced) immediately to the disintegrator, then the adhesion and clogging to such as screen in the disintegrator tend to be caused, also because the high-concentration hydrogel sheet has just been produced and is therefore in a high-temperature state.
As to the cutting art of the patent document 2 above, because the rectangular sheets having been cut by the one pair of roller type cutters come straight down onto the revolution body as provided with the rotary blade, the small pieces having been cut by the rotary blade and the fixed blade tend to remain adhering to the surface of the revolution body. If the small pieces accumulate on the outer periphery of the revolution body, then the cutting operation unfavorably becomes impossible.
In the patent document 2 above, it is proposed that the temperature should be dropped by cooling the gel sheet down to the range of about 10 to about 30° C. in order to prevent the adhesion. However, in order for the gel sheet (having a possibility of coming into a high-temperature state near 100° C. just after having been produced) to sufficiently be cooled no later than being fed to the cutter, it is necessary that the gel sheet is run a long distance from its production device to the cutter, or that a cooling device is added on the running course, or that the running speed is made slow to thus secure the cooling time. Therefore, costs of facilities increase, and the production efficiency deteriorates.
As to the cutting art of the patent document 3 above, because the continuous sheet is cut in such a manner that it is scraped off with the plate-shaped rotary blade by making this blade bite into the continuous sheet at a stroke in the nip clearance between rollers, it is difficult to surely cut the whole thickness of the continuous sheet. Particularly, in the case of such a high-concentration hydrogel sheet as has the dispersion in thickness or the thicknesswise undulating or waving, its cutting is difficult. Because the continuous sheet is run in a state nipped (i.e. interposed) between the cutting roller and the opposite roller, the continuous sheet tends to adhere to surfaces of the cutting roller and opposite roller. Particularly, the cut gel pieces result in tending to unfavorably adhere to the opposite roller.